On Thu, 23 Jun 2005, Malcolm Wallace wrote:
module Fieldbug where data Fields a = VariantWithTwo { field1 :: a , field2 :: a } | VariantWithOne { field1 :: a }
The key point here is that the data structure with named fields has more than one constructor, and some fields are omitted in one of the variants. Now let's try to write a simple conversion function over values of this type, using only the named-field update style:
data Void = Void
voidcast :: Fields a -> Fields Void voidcast v@(VariantWithTwo{}) = v { field1 = Void , field2 = Void } voidcast v@(VariantWithOne{}) = v { field1 = Void }
It looks simple enough doesn't it? But none of the widely available Haskell implementations will accept this code.
This is either a point against multiple fields with the same label or against the field update syntax. Against multiple declarations because it is not allowed to define multiple objects with the same name in the same scope. But if you consider multiple occurences of the same field name as distributed declarations of the same function the conflict is resolved. So it seems to be more a point against the record update syntax. What I need quite often is a function which updates a field. This would nicely work also for your example. update_field1 :: (a -> a) -> Fields a -> Fields a update_field1 f (VariantWithTwo x y) = VariantWithTwo (f x) y update_field1 f (VariantWithOne x) = VariantWithOne (f x) (update_field1 (const x)) would replace a field value with a new one. Support for this kind of update would be better than support by the current update syntax.